The present disclosure relates to a fixing device fixing a toner image onto a recording medium and an image forming apparatus including the fixing device.
Conventionally, an electrographic image forming apparatus, such as a copying machine or a printer, includes a fixing device fixing a toner image onto a recording medium, such as a sheet. In the fixing device, a heat roller manner is widely applied. The heat roller manner is a manner making a pair of rollers come into pressure contact with each other to form a fixing nip. However, in order to sufficiently ensure a nip width (a width of a fixing nip) by the heat roller manner, it is necessary to enlarge an external diameter of each roller forming the fixing nip, and accordingly, heat capacity of components forming the fixing nip is increased. Because of this, a restoring time of the fixing device (a time until it is restored to a condition capable of fixing the toner image onto the sheet) is elongated to obstruct energy saving. If the external diameter of each roller forming the fixing nip is enlarged, a problem of deteriorating separability of the recording medium from each roller is brought.
Recently, in order to solve the above-mentioned problem, a belt fixing manner is applied. The belt fixing manner is a manner making a fixing belt and a pressuring member (e.g. a pressuring roller) come into pressure contact with each other to form a fixing nip. By applying the belt fixing manner, the nip width may be sufficiently ensured without enlarging external diameters of the fixing belt and pressuring member.
For example, there is a fixing device including a fixing belt arranged rotatably, a pressuring member coming into pressure contact with the fixing belt to form a fixing nip, a tensioning roller arranged to sandwich the fixing belt with the pressuring member and a slide contact member arranged to sandwich the fixing belt with the pressuring member.
In such a fixing device, the fixing belt is stretched across the tensioning roller and slide contact member and constant tension strength is applied to the whole area of the fixing belt. In such a condition, when the fixing belt is rotated, a sliding load of the fixing belt (a load required for sliding the fixing belt with respect to the slide contact member) becomes large. According to this, there is a possibility that the fixing belt is rapidly worn and durability of the fixing belt is deteriorated. If the fixing belt is rotated in a condition having the large sliding load, and is repeatedly bent in accordance with a shape of the slide contact member, there is a possibility of bringing damage of the fixing belt.
In order to rotate the fixing belt in a condition where the constant tension strength is applied to the whole area of the fixing belt, it is necessary to increase a nip pressure (a pressure of the fixing nip), and accordingly, drive torque of the fixing belt (torque required for rotating the fixing belt) is increased. If the nip pressure is increased as mentioned above, because each component of the fixing device must be enlarged in order to ensure strength withstanding the increased nip pressure, there is a possibility of bringing enlargement of the entire fixing device. In addition, if the nip pressure is increased as mentioned above, because the force shifting the fixing belt to one side in a longitudinal direction is strengthened, there is a possibility that an end part of the fixing belt interferes with other components and is damaged.